1. Field of the Invention
The present invention relates to a method of manufacturing a power semiconductor device, and particularly, to a method of manufacturing a power semiconductor device having a trench gate.
2. Description of the Background Art
There are some power semiconductor devices each used as a contactless switch for controlling a large amount of electric power. Such a device for a large amount of electric power is applied, for example, to the inverter circuit of household electric appliances such as an air conditioner, a refrigerator and a washing machine which have implemented improved energy conservation, and to the motor control for trains such as a bullet train and a subway train. In recent years, in consideration of the global environment, the power semiconductor device is applied for controlling the inverter and converter of a hybrid vehicle which runs using a combination of the electric motor and the engine, and also applied to a converter used for solar photovoltaic power generation or wind power generation. In this way, the application field of the power semiconductor device has been enlarged.
Furthermore, there are also some power semiconductor devices for controlling a small amount of electric power. Such a semiconductor device includes, for example, a power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) for a small amount of electric power. Such an MOSFET serves as a unipolar device which is suitable for high-speed switching, and accordingly, used for mobile phones and game machines, for example. Furthermore, particularly in the low-withstand-voltage type power MOSFET, finer pattern formation causes an increase in channel width and a high current density, which allows a decrease in the resistance (on-resistance) between the drain and the source during the passage of current.
However, since there is a limit to the finer pattern formation, a trench gate structure may be used for further decreasing the on-resistance of the low-withstand-voltage type power MOSFET. The power MOSFET having a trench gate as described above and the manufacturing method thereof are disclosed, for example, in FIGS. 14-31 of Japanese Patent Laying-Open No. 09-246550.
Furthermore, the trench gate is used not only for an MOSFET but also for an IGBT (Insulated Gate Bipolar Transistor). Such an IGBT is disclosed, for example, in Japanese Patent Laying-Open No. 2007-311627.
The power semiconductor device is required to improve the performance and lower the chip cost. A reduction in chip size is effective in lowering the chip cost. However, in the power semiconductor device for a small amount of electric power not more than 1 A which is in high demand and gains a large market in the field of mobile phones and mobile tools, the chip size has already reached its lower limit, which makes it difficult to further reduce the chip size. Therefore, there is a need for an approach other than that of reducing the chip size to lower the chip cost, and also a need to simplify the manufacturing method as one of the approaches.